1. Field of the Invention
The present invention relates to a method and system for generating medical images. Particularly, the present invention is directed to a method and system for generating magnetic resonance (“MR”) images of a patient.
2. Description of Related Art
A variety of systems and related methods are known in the art for generating magnetic resonance images of a patient. Currently, whole body MRI systems utilize, among other things, cylindrical superconducting magnets to facilitate imaging of the entire human body using linear moving patient tables and magnets with large homogeneous volumes. The inner diameter typically accessible to the patient is from about 600 mm to about 700 mm. The magnets are typically 1.75 to 2.25 meters long. Some systems have magnets as short as 1.25 meters. The typical imaging volume is 350 mm to 450 mm in each of three dimensions with the shorter systems having smaller homogeneous volumes. As known in the art, references herein to such homogeneous volumes refer to regions of sufficient magnetic field uniformity to facilitate creation of useful MRI images.
Some extreme engineering design measures can be taken whereby the amount of superconducting wire in the main magnet is greatly increased in attempts to improve the size of the homogeneous volume at shorter magnet lengths. However, this can only help to a limited extent. The physics and engineering limits require a reduction in homogeneous volume as the magnet is made shorter.
The hip and shoulder joints are off patient center in left/right directions during imaging. The preferred imaging field-of-view for imaging these joints is about 160 mm or less for an imaging volume centered about the joint. However, the magnet's useful imaging volume must be larger to place the desired joint in a good portion of the imaging volume even though the necessary imaging field-of-view is substantially less than the overall imaging volume of the system. An off center imaging sequence is prescribed resulting in an image acquisition centered about the joint.
Along with a large imaging volume in such devices, there is also a requirement to create an RF transmit magnetic field and gradient magnetic field that that spans the entire imaging volume. These coils are generally cylindrical occupying the volume between the magnet and patient bore for a full 360 degrees surrounding the patient.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, such systems are necessarily large. The larger the system, the larger the main magnet needs to be to surround the patient volume, gradient and RF coils. These larger gradient coils, in turn, require more power. Extra space in the facility along with a larger Radio Frequency shielded room is also required. The main magnet is mainly made using low temperature superconductors (“LTS”) such as niobium-titanium and the like. These materials are expensive. All of these factors increase the system costs. As such, there is a continuing need in the art for more suitably designed imaging systems that minimize these and other drawbacks. The present invention provides a solution for these and other problems.